Has anyone already toyed with the idea of making the amount of "on screen" time a character gets a metagame characteristic, independent of the character's power level or the player's theatrical skill/shyness?

The closest example I can think of is that in Puppetland, each session takes a real-world hour. The GM actually sets an alarm clock. But, the *characters* know the session will take a real-world hour. So when the GM narrates a long trek across the tablecloth, if his description takes 2 minutes to describe a long period of walking, then 2 minutes pass, and the alarm clock keeps ticking.

This tends to mean the sessions comes to a crescendo as the characters rush around, realising they're running out of time.

I'm very intrigued, though, how would you see this characteristic working?

So you want an example? Mmm...here goes: In the funny independent French RPG Bioman ( a spoof of Japanese live action superheroes show), characters have a characteristic named "stupefaction", that is used to find out who gets to be "on camera" at a given time (because in the show, it's always the most flabbergasted character who does); The camera (hence play focus) stays on him until he does nothing, or another character does something more exciting; another way to be on camera is...to stand behind the stupefied character !!

I laughed my ass off reading the game, but later went "mm...some potential here". Hence my question. It would be a cool way to protagonize shy or beginner players, and to avoid campaigns where the highest level character always steals the show because of his powers.

Has anyone already toyed with the idea of making the amount of "on screen" time a character gets a metagame characteristic, independent of the character's power level or the player's theatrical skill/shyness?

Yes. I'm in the process of writing a game that does that very thing. I call it Influence and I should be posting a link to the first version any day now. Basically players are bidding for different roles - each of which awards them with a role for the upcoming scene. Roles are divided up into three categories: main characters, supporting cast, and background cast. The closer you are to a main character the more screen time you receive, and the more narrativist power you get. I'm hoping that in actual play the bidding system will give everyone a chance to take a shot at the more powerful roles, but I have a feeling that the restrictions that I've placed on the less influencial roles will have a larger effect on a player's desire to become a main character. Anyway, when I post the game I'll give a bit more information.

Make sure to check out any of the games based on TV or movies as a medium, as opposed to just as content. It Came from the Late Late Late Show, Extreme Vengeance, Hong Kong Action Theater, and Tales from the Crypt ... probably others. Many of them have a literal "I'm on screen" mechanic of some kind, even including the ability to upstage other characters.

Don't forget about The Framework. It deals with a shiting spotlight, independent of the character's power level.

However, like most games in the same vein, the mechanic still requires a certain amout of initiative on the players part. So if you have a player who is always shy, the game isn't going to force them to play a larger role.